You're eating dinner after a hard day's work, and the phone rings. It's a telemarketer. "Hey, I'm on the 'Do Not Call' list, so stop calling me," you say. He laughs and keeps calling, every 10 minutes, day and night, forevermore. Sound like a nightmare? Maybe not.

What keeps telemarketers from calling day and night? It's not just that there are laws against it; it's that the telemarketers can be identified and made subject to those laws. Suppose telemarketing invaded the VoIP world, creating "vam" instead of spam? What's to stop a telemarketer using VoIP from spoofing a calling address the same way spammers spoof e-mail addresses? Why couldn't a big server in some offshore haven generate zillions of Session Initiation Protocol (SIP) calls with these spoofed addresses?

What kind of VoIP we get is likely to depend on how pushy we are about the problems of vamming. The time has come to recognize that Internet freedom means letting people protect themselves and to look more closely at VoIP service technology.

The standard for VoIP, SIP, establishes an open model where users have IP phones linked to the permissive Internet infrastructure. In theory, anyone who can locate such a phone via scanning can call it, and with the cost of an Internet call near zero, this model invites vamming as soon as the community of open VoIP users gets large enough to exploit. Fortunately, this isn't a popular model among VoIP providers.

What is popular is a closed model, some version of which nearly every VoIP provider uses today. Under it, users have IP phones that are in some way isolated from the open and uncontrolled Internet community. Some providers, such as Skype, are partitioning through the use of proprietary protocols and encryption; others, such as Verizon, are looking at doing VoIP over a true VPN. Either of these approaches would limit access to VoIP customers by outsiders, including telemarketers.

In a closed model, users can be authenticated, which makes enforcing civil or criminal penalties possible. In theory, VoIP users in these closed systems could demand that their carriers filter out calls from sources not subject to U.S. telemarketing laws. If all members of a VoIP community are identified, and called parties can refuse calls that are not subject to prevailing telemarketing or other consumer legislation, it would work like the public switched telephone network.